Traction control system

ABSTRACT

A brake system for a vehicle having a traction control or anti-spin function for controlling wheel slip on differentially driven wheels of a vehicle. The system includes left and right fluid pressure operated brake mechanisms associated with left and right driven wheels of the vehicle. First and second brake valves control the communication of fluid pressure to the left and right brake mechanisms. In normal operation, an operator actuatable treadle concurrently operates both valves to simultaneously apply fluid pressure to the brake mechanisms as a function of treadle depression. Each brake valve includes a pilot pressure operated section for actuating a main fluid control section. Wheel speed sensors associated with the left and right driven wheels continuously monitor the rotational speed of the wheels. When a slip condition is detected, a control unit connected to the wheel sensors activates one of two pulse width modulated valves to produce a pilot pressure in a pilot pressure chamber of the brake valve associated with the faster rotating wheel. The communication of pilot pressure to the brake valve causes actuation of the brake mechanism thereby applying braking torque to the faster rotating wheel to reduce its wheel speed. Engine torque is thus transferred via the differential to the other wheel.

TECHNICAL FIELD

This invention relates generally to vehicle drive systems and, inparticular, to a traction control or anti wheel spin system for use withdifferentially driven wheels.

BACKGROUND ART

In a typical vehicle to which this invention pertains, a pair of wheelsare driven through a differential which divides and applies enginetorque to the wheels attached to the differential by way of axles. Inconventional drive trains such as those found on highway and off-highwayvehicles, the differentials operate to transfer torque to the wheelhaving the lowest traction. Limited slip and locking differentials areused on some vehicles to provide a means for transferring engine torqueto the wheel with the most traction. Limited slip and lockingdifferentials, however, can be very expensive and, in the case oflimited slip differentials, may not be sufficiently effective totransfer torque to a wheel sitting on an extremely low traction surface,such as ice.

More recently, traction control systems which utilize the braking systemto control wheel slip have been introduced. In many, if not most ofthese systems, when wheel slip is detected, the brakes of the fasterrotating wheel are applied to reduce its speed to that of thenon-slipping wheel. Many of these systems are complex and addsignificant cost to the vehicle.

DISCLOSURE OF THE INVENTION

The present invention provides a new and improved traction controlsystem which is operative to actuate the brakes on a faster rotatingwheel when a slip condition is sensed.

In the illustrated embodiment, the invention is described in connectionwith a brake retarder system forming part of an off-highway vehicle. Itshould be understood, however, that the principles of the invention canbe applied to a conventional brake system.

According to the invention, the brake retarder system with the tractioncontrol function comprises first and second treadle operated brakevalves for controlling the flow of the pressurized fluid to at least onebrake assembly associated with each driven wheel. An operator actuatedtreadle is operatively coupled to both of the treadle operated valves sothat movement of the treadle produces concurrent movement in the valves.When operated in this mode, depression of the retarder treadle causessubstantially equal fluid pressures to be concurrently conveyed to thebrake assemblies of both driven wheels, with the fluid pressure being afunction of treadle depression.

Each treadle operated valve includes a pilot pressure operated elementand a main control element. Force is applied to the main control elementto produce the communication of pressurized fluid to the associatedbrake assembly with the fluid pressure communicated being a function ofthe force applied to the valve element. In normal operation, operationof the treadle by the operator produces the force on the main valveelement which results in the communication of fluid pressure to thebrake assemblies that is a function of treadle depression.

The pilot pressure operated element is operative to apply forces to themain valve element when pressurized fluid is communicated to theelement. A modulating valve arrangement including preferably twomodulating valves associated with the first and second pilot pressureoperated elements are operative to pressurize a pilot pressure chamberunder predetermined operating conditions. In particular, a tractioncontrol unit monitors the rotational speeds of the left and right drivenwheels. When the speed of one wheel exceeds the other wheel by apredetermined level, the traction control unit recognizes this to be aslip condition and in response, actuates the modulating valve associatedwith the brake valve connected to the brake assembly of the fasterrotating wheel. The brake assembly is actuated to apply a braking torqueto the faster rotating wheel. In the preferred arrangement, the amountof pressure generated by the modulating valve is a function of a wheelslip signal, i.e., a signal that is related to the difference inrotational speeds between the left and right driven wheels.

In the preferred and illustrated embodiment, the modulating valves areeach pulse width modulated valves. The traction control unit uponsensing a slip condition generates a pulse width modulated signal toactuate the valve associated with the faster rotating wheel.

In the preferred and illustrated embodiment, the traction control systemis activated when a wheel slip difference of 30% is detected. In a morepreferred embodiment, a control actuatable by the operator can decreasethe sensitivity so that the traction control system is activated whenthe wheel slip difference is only 10%. Preferably, the control is a pushbutton.

In the preferred system, the fluid pressure generated by the pulse widthmodulated valves, form part of a circuit in which fluid pressure iscontinually bled to tank via an orifice. The orifice size is selectedsuch that fluid pressure is maintained upstream of the orifice whenfluid flow above a predetermined level is maintained. The pulse widthmodulated valve is located upstream of the orifice and when activatedcontrols the amount of fluid pressure communicated to the passage orconduit that communicates with the orifice. A pilot pressure passagecommunicates the fluid pressure between the orifice and the pulse widthmodulated valve to the associated pilot pressure chamber. Thus, thelevel of pressure in the pilot pressure passage is a function of theduty cycle under which the pulse width modulated valve is operated. Whendeactivated, the pulse width modulated valve returns to a fluid blockingposition so that pressure from the source is blocked from the orifice.Consequently, pressure in the orifice and the pilot pressure passage isdischarged to tank. In this way, fluid communicated to the pilotpressure chamber is exhausted without the need for a separate returnline.

Additional features will become apparent and a full understandingobtained by reading the following detailed description made inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a traction control systemconstructed in accordance with the preferred embodiment of theinvention;

FIG. 2 is a schematic representation of the fluid pressure operatedportion of the traction control system;

FIG. 3 is a sectional view of a treadle operated valve which may be usedin the system shown in FIG. 2; and,

FIG. 4 is a fragmentary view, partially in section, illustrating a wheelspeed sensor constructed in accordance with the preferred embodiment ofthe invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically illustrates a traction control system constructedin accordance with the preferred embodiment of the invention. The systemis intended to control wheel slip between differentially driven wheelsof a vehicle. In the illustrated embodiment, the vehicle includes anaxle 10 which houses a differential 12. The differential 12 is operativeto divide and apply engine torque to outboard driven wheels 14, 16 bymeans of respective axle shafts 18, 20. The differential, wheels andaxle shafts are considered conventional and do not form part of theinvention. As is also conventional, rotation in the wheels 14, 16 isarrested by associated brake mechanisms 22, 24, respectively (shown inFIG. 2). The brake mechanisms may comprise shoe brakes, caliper discbrakes, or single or multi wet disc brakes.

As seen in FIG. 1, the rotational velocities of the wheels 14, 16 aremonitored by respective speed sensors 30, 32 which in the illustratedembodiment separately monitor the rotational speeds of the axles 18, 20.A controller 40 monitors the wheel rotational speeds and upon detectingwheel slip develops a "slip" signal which is at least partially, afunction of the difference in the rotational speeds of the wheels 14,16. The "slip" signal in turn is used to generate a actuating signal forcausing a brake control 46 to apply the brake mechanism of the fasterrotating wheel in order to reduce its rotational speed.

In the preferred embodiment, the traction control system forms part of abrake retarder system which is schematically illustrated in FIG. 2. Asis known, some vehicles such as heavy duty on-highway and off-highwayvehicles such as large haulers, include a separate brake control systemwhich provides a "retarder" function. Generally, the retarder system isactuated by the vehicle operator by pressing on a separate retardertreadle which effects actuation of the rear wheel brakes only. Generallythis feature is used when the vehicle is traveling down a gradual butlengthy grade. Although the invention is being described in connectionwith a vehicle retarder system, it should be understood that it can alsoform part of a conventional brake system.

The brake retarder system shown in FIG. 2 is operative to control theapplication of pressurized fluid from a source 50 to the fluid pressureoperated brake assemblies 20, 22. In the illustrated embodiment, thebrake assemblies 20, 22 may comprise multi disc wet disc brakes such asthat shown in U.S. Pat. No. 4,562,902. The invention, however, can beused with other types of fluid pressure operated brakes.

In normal operation, fluid pressure is concurrently communicated to thebrake assemblies 20, 22 in response to the depression of a treadle orpedal 54. The brake actuation circuit for brake assemblies 20, 22includes respective shuttle valves 60, 62. The shuttle valve 60 controlswhich of two pressure supply lines communicate with a brake conduit 66that is connected to the brake assembly 22. The shuttle valve 62controls which of two pressure supply lines communicate with a brakeconduit 68 that is connected to the brake assembly 24. As seen best inFIG. 2, the shuttle valve 60 communicates with pressure supply lines 70,72, whereas the shuttle valve 68 communicates with pressure supply lines74, 76. Turning first to shuttle valve 60, the supply line 72 receivespressurized fluid from the brake retarder system whereas the supply line70 receives pressure from a conventional brake system. A line 78 whichis connected to the conventional brake system and which receives fluidpressure under control of a conventional brake treadle valve,communicates pressure concurrently to the supply line 70 and the supplyline 74. The supply lines 74, 76 receive fluid pressure from theretarder brake system, the pressure being a function of retarder pedaldepression, under normal operation. It should be apparent, that theshuttle valves 60, 62 communicate the pressure supply line having higherpressure to the associated brake assembly. Thus, when the conventionalbrake pedal (not shown) is operated, the shuttle valves 60, 62 open toallow pressurized fluid to flow into the conduits 66, 68 from theconduits 70, 74, respectively while blocking communication with theretarder pressure supply lines 72, 76. Conversely, when the retarderpedal 54 is depressed, pressurized fluid from the retarder system iscommunicated to the individual brake actuating conduits 66, 68 by theshuttle valves 60, 62 which operate to block communication with thepressure supply lines 70, 74.

The overall brake operating system including the traction controlfunction can be divided into two subsystems, namely the retardersubsystem 46a and the traction control energization subsystem 46b.

The retarder subsystem 46a includes a pair of brake pressure controlvalves 80, 82 which control the communication of pressurized fluid tothe retarder pressure supply line 72, 76, respectively. In normaloperation, the control valves 80, 82 are concurrently actuated by thetreadle 54.

The brake valves 80, 82 include main valve elements 80a, 82a which areconnected to the respective pressure supply lines 72, 74. The mainelement 80a also communicates with the source of pressurized fluid 50via branch conduit 100, branch conduit 102 and main pressure conduit104. The main valve element 80a also communicates with a tank 106through branch conduit 108 and main tank conduit 110. When the mainelement 80a is in the non-actuated position shown in FIG. 2, thepressure supply line 72 is communicated with the tank 106, so that anypressure in the pressure supply line 72 is discharged thus, releasingthe brake 22. The valve 82 includes a main element 82a whichcommunicates with the source of pressurized fluid through a branchconduit 112, the branch conduit 102 and the main conduit 104. The mainelement 82a also communicates with the tank 106 via branch conduit 112and main tank conduit 110. The main element 82a functions the same asthe main element 80a.

In normal operation, when the treadle 54 is depressed by the operator,operating springs 120, 122 are compressed by respective pistons 124,126. The compression of the springs 120, 122 causes an opening force tobe applied to the main elements 80a, 82a. The application of the springforce to the main elements, causes them to move towards an open positionallowing pressurized fluid to flow from the branch conduits 100, 112 tothe pressure supply lines 72, 76, respectively. Fluid pressure in thesupply line 72 is fed back to an effective pressure area on the mainelement 82a (the feedback circuit is represented by a passage 72a) whichcreates a force that opposes the spring force exerted by the spring 120.Eventually, the fluid pressure in the supply line 72 will generate asufficient opposing force on the main element 80a to terminatecommunication between the branch pressure conduit 100 and the supplyline 72. Thus, a pressure corresponding to the spring force applied tothe main element 80a, which in turn, is related to the extent ofdepression of the treadle 54, will be maintained in the brake actuationconduit 66, which in turn will cause the brake assembly 20 to apply abraking torque to the wheel, which again is a function of treadledepression. When the treadle is released, the pressure supply line 72 iscommunicated with the branch tank conduit 108 and pressure in theconduit 66 is discharged, thus releasing the brake 22.

The main element 82a operates in the same manner to actuate the brakeassembly 24 for the other driven wheel 16.

Each valve 80, 82 also includes a pilot pressure operated element 80b,82b which include the pistons 124, 126. Respective pilot pressurechambers 130, 132 are defined above pistons 124, 126 of the controlvalves 80, 82. In normal retarder operation, movement in the pistons124, 126 is effected by respective push rods 132, 134 which are coupledto the treadle 54 for joint motion. The pistons 124, 126 are notattached to the push rods 132, 134 and can move independently under theinfluence of fluid pressure communicated to the associated fluidchambers 130, 132.

More specifically, the spring 120 can be caused to apply a biasing forceto the element 80a by communicating fluid pressure to the chamber 130.The communication of fluid pressure to the chamber 130 of the valve 80causes the piston 124 to move downwardly, thus compressing the spring120 and thereby exerting a biasing force on the valve element 80a. Asthe valve element 80a opens under the influence of the spring force,pressurized fluid is communicated to the brake actuating conduit 66 viathe supply conduit 72. The fluid pressure communicated to the conduit 66is a function of the amount of pilot pressure communicated to thechamber 130 and is entirely independent of the pressure, if any,communicated to the actuating conduit 68 by the control valve 82.

The valve 82 also includes a pilot pressure operated section 82b whichfunctions just as the section 80b, except that the communication ofpressurized fluid to the chamber 132 causes pressurized fluid to becommunicated to the actuating conduit 68 of the brake assembly 22. Withthe illustrated embodiment, the brake assemblies 20, 22 can beindependently actuated by selectively communicating predetermined pilotpressures to the respective pilot pressure chambers 130, 132.

The communication of pilot pressure to the chambers 130, 132 iscontrolled by the traction control subsystem 46b. The subsystem 46bincludes a pair of pulse width modulated (PWM) valves 140, 142 whichcontrol the communication of pressurized fluid to respective tractioncontrol supply conduits 150, 152. It should be noted, as illustrated inFIG. 1, that the retarder subsystem 46a and the traction controlsubsystem 46b can form a single assembly or manifold in the vehicle.

The supply conduits 150, 152 communicate with respective pilot pressurechambers 130, 132 via pilot passages 154, 156. The PWM valve 140controls the pressurization of the pilot pressure chamber 130 of thevalve 80, whereas the PWM valve 142 controls the pressurization of thepilot pressure chamber 132 of the control valve 82. The PWM valvesconcurrently communicate with a source of pressurized fluid which, inthe illustrated embodiment includes a regulator 170 and a supply conduit172. The traction control supply conduit 172 communicates with the mainpressure conduit 104.

With the PWM valves in the positions shown in FIG. 2, fluidcommunication to the traction control supply conduits 150, 152 isblocked. The traction control supply conduit 150 concurrentlycommunicates with the tank 108 via a return conduit 184 and an orifice186. The size of the orifice is selected such that it restricts the flowof pressurized fluid so that a pressure is developed on the upstreamside of the orifice 186. Any pressure developed in this segment of theconduit is communicated to the pilot pressure chamber 130 by the pilotpassage 154. When the PWM valve is in the position shown in FIG. 2, anypressure in the pilot pressure chamber 130 is discharged to the tank 106via the conduit 150, the orifice 186 and the return conduit 184. Thetraction control feed conduit 152 is similarly arranged and communicateswith the tank 106 via return conduit 188 and orifice 190. Any pressuredeveloped in the conduit 152 due to the action of the orifice 190 iscommunicated to the pilot pressure chamber 132 via pilot passage 156.Pressure in the chamber 132 is communicated to the tank when the PWMvalve 142 is in the position shown in FIG. 2. The PWM valve 142 providesthe same function as the PWM valve 140 except that it controls thepressurization of the pilot pressure chamber 132 of the valve 82.

In operation, the controller 40 produces an actuating signal undercertain conditions which actuates one of the PWM valves in order todevelop pressure in one of the pilot pressure passages 154, 156. Theamount of pressure developed in a given passage is determined by theduty cycle under which the PWM valve is actuated by the controller.

The controller 40 monitors the rotational speed of the wheels 14, 16 bymeans of the wheel speed sensors 30, 32. As is known, when a vehicle ismoving in a straight line, the wheel speeds of the wheels 14, 16 shouldbe substantially equal. When turning a corner, however, the inside wheelwill rotate at a speed that is less than the wheel speed of the outsidewheel. The controller 40 is programmed to recognize when the differencein wheel speeds between the wheels 14 and 16 is of a level that wouldindicate that the vehicle is merely making a turn. However, when thewheel speed of a given wheel is substantially greater than the oppositewheel, indicating that the faster rotating wheel is in fact slipping dueto a low traction surface, the controller 40 recognizing this condition,immediately activates the PWM valve that controls the communication ofactuation pressure to the brake assembly associated with the fasterrotating wheel. Activation of a given PWM valve such as the valve 140establishes a pilot pressure in the pilot passage 154 therebypressurizing the associated pilot pressure chamber 130. Pressurizationof the chamber 130 causes the piston 124 to apply a biasing force to themain section 80a whereby fluid pressure is communicated to the brakemechanism 22. The application of brake torque to the wheel 14 reducesits rotational speed. Since the left and right wheels are driven througha differential, the application of braking torque to the faster rotatingwheel causes the transfer of at least some engine drive torque to theother driven wheel.

In the preferred embodiment, the duty cycle under which the PWM valve isactuated is a function of the difference in wheel speed which may betermed a slip signal. The pressure developed in the pilot pressurechamber is preferably function of or is proportional to the magnitude ofthe slip signal. When the wheel speed of the faster rotating wheel isreduced to a predetermined level with respect to the other wheel, theassociated PWM valve is deactivated and thus allows the pressure in thepilot pressure chamber to flow to the tank through the associatedorifice and the associated return conduit thus releasing the brakemechanism.

In the illustrated system, the traction control subsystem is activatedwhen the controller 40 detects that one wheel is rotating 30% fasterthan the other wheel. When the wheel speed difference is less than 30%,the controller assumes that the vehicle is turning and the tractioncontrol subsystem is not activated. According to a feature of theinvention, however, the operator is given the option to increase thesensitivity of the system. This is achieved in the illustratedembodiment by a push button 210 which, when depressed, forces thecontroller 40 to activate the traction control subsystem when the wheelspeed difference exceeds 10% (as opposed to 30%). This feature allows anoperator to increase the sensitivity of the system when the vehicle isbeing operated in a straight line and in a difficult traction situation.

The system also includes an error light 220 which illuminates when anerror, i.e., sensor problem or other fault is detected. It should benoted here that the controller 40 may take several forms. Preferably,the controller is microprocessor based and the functions it performsrelated to monitoring wheel speeds and the generation of the wheel slipsignal, as well as the signals needed to activate the pulse widthmodulated valve associated with the faster rotating wheel can beproduced using hardware or software or combinations of both. Controllerscapable of performing the disclosed operation and function are availablefrom APITECH, a division of Applied Power, Inc. located in Butler, Wis.

FIG. 3 illustrates the construction of a pressure control valve whichmay be used to provide the function performed by the valves 80, 82 shownschematically in FIG. 2. The valve illustrated in FIG. 3 has in thepast, been used for other brake applications. It should also beunderstood that other types of valves may be used to serve the functionperformed by the valves 80, 82.

To facilitate the explanation, the valve will be described as though itwere the pressure control valve 80 shown in FIG. 2. Components in thevalve which are identical or similar to the elements shown schematicallyin FIG. 2, will be given like reference characters.

The valve assembly 80 shown in FIG. 3, is arranged in a cartridgeconfiguration which, as is known, is intended to be mounted within acavity having a multi-stepped bore (not shown). O-ring grooves 200, 202,204 and O-ring shoulder 205 formed on the valve housing carry O-rings200a, 202a, 204a, 205a which sealingly engage sections of the cavity andserve to isolate regions of the valve indicated generally by thereference characters 206, 208, 210, 212. The cavity into which the valve80 is mounted includes passages for communicating fluid pressure to andfrom the isolated sections of the valve.

The valve includes a plurality of supply pressure ports 214 whichcommunicate with the conduit 100 (shown in FIG. 2). The housing alsodefines a plurality of working or brake ports 216 which communicate withthe conduit 72 (shown in FIG. 2). A plurality of tank ports 218communicate with the return line 108 (shown in FIG. 2). Finally, a pilotpressure bore 154a is formed near the top of the valve and forms atleast part of the pilot passage 154 shown in FIG. 2.

Extending from the top of the valve assembly is the push rod 132 whichis suitably connected to a treadle 54 (shown in FIG. 2). As seen in FIG.3, downward motion of the push rod 132 causes downward movement in thepiston 124 which, in turn, effects downward movement in a spring seat220.

The spring 120 shown schematically in FIG. 2, is a coiled compressionspring 120 that is captured between the spring seat 220 and areciprocally moveable spring guide 222. It should be apparent thatdownward movement of the piston 124 causes the spring 120 to exert abiasing force on the spring guide 222, urging it downwardly.

The spring guide 222 is operatively coupled to a fluid control spool 224having an upper land 226 and a lower land 228 interconnected by a narrowdiameter section 230. The lower land 226 controls the communication offluid pressure from the pressure supply ports 214 to the brake ports216. The fluid control spool 224 is biased upwardly by one or morebiasing coil springs 232 captured between a lower end face 228a of thelower land 228 and an end face of a blind, substantially sealed bore236. When sufficient spring force is applied to the spring guide 222 toovercome the spring force exerted by the biasing spring 232, the fluidcontrol spool 224 moves downwardly until at least a portion of thesupply ports 214 are uncovered thereby allowing fluid flow into thebrake supply ports 216 via the gap defined by the narrow diametersection 230 of the spool 224.

Fluid pressure communicated to the brake ports is also communicated tothe blind bore 236 via a radial passage 240 and an axial passage 242.Together these passages define the feedback passage shown schematicallyand labeled 72a in FIG. 2. The communication of brake pressure to thelower end face 228a of the spool 224 creates a force urging the spoolupwardly. The fluid pressure generated force on the lower end face 228aof the lower land 228 will eventually overcome the spring force exertedby the spring 120 and move the fluid control spool 224 upwardly untilthe supply ports 214 are sealed from the brake ports 216. At this point,no further fluid pressure is communicated to the brake ports 216 and aconstant fluid pressure is maintained in the brake actuation conduit 66as long as the piston 124 and hence the spring seat 220 remainsstationary.

If the treadle is released thereby allowing the piston 124 and springguide 220 to move upwardly, the release or reduction of spring force onthe fluid pressure control spool 224 allows it to move upwardly until afluid communication is established between the brake ports 216 and thetank ports 218 whereby fluid pressure in the conduit 66 is discharged.If the actuating rod is allowed to move to its fully released position,substantially all fluid pressure in the brake ports 216 and brakeconduit 66, will be discharged. However, if the actuating rod 132 isonly partially released, fluid pressure will be discharged until a lowerpressure is reached corresponding to the position of the treadle, atwhich time further discharge of pressure ceases and a lower fluidpressure is established in the conduit 66.

As seen in FIG. 3, a series of longitudinal clearance slots 250 areformed on the periphery of the upper land 226 to improve performance.The purpose of the clearance slots 250 is to allow fluid trapped in theblind bore 236 to escape to tank during the initial downward movement ofthe fluid control spool 224. Substantially smaller slots 252 are alsoformed in the lower land 228, the purpose of which is to reduce noiseduring operation. The slots 252 effectively meter a small amount offluid to the brake ports 216 as the lower land 228 moves to the openposition. With the slots 252, the lower land 228 does not operateabruptly as a fully "on" and fully "off" control.

The three circumferential grooves 260 formed on the lower land serve asbalancing grooves which aid in the centering of the land 228 within thehousing bore. O-ring 262 is used to seal the piston 124 and inhibitleakage out of the chamber 130. In the illustrated embodiment, the valvealso includes an auxiliary spring 264 which acts only against the springseat 220 thereby applying a return force to the spring seat 220 and,hence, the piston 124 independent of the biasing spring 120.

The traction control operates as follows. When the traction control isactive and the wheel speed of the wheel 14 exceeds the wheel speed ofthe wheel 18 by at least 30%, and the control unit recognizes that thebrake mechanism 22 requires actuation to reduce the rotational speed ofthe wheel 14. The control unit 40 generates an pulsed actuation signalfor the pulse width modulated valve 140. Actuation of the PWM valve 140communicates pressure to the chamber 130, via the pilot pressure passage154a formed in the valve 80 (see FIG. 3). The communication of pressureto the chamber 130 urges the piston downwardly, independently of thepush rod 132 thus applying a biasing force to the fluid control spool224 and ultimately causing he communication of fluid pressure from thesupply line 214 to the brake ports 216 and hence the brake actuationconduit 66. The pressure established in the conduit 66 is a function ofthe pilot pressure established in the chamber 130. As explained above,with the pulse width modulated valve 140 ceases operation, pressure inthe supply conduit 150 is discharged to tank 106 via the orifice 186 andany fluid pressure in the chamber 130 is also discharged, thus fullyreleasing the brake assembly 22.

FIG. 4 illustrates a wheel speed sensor 300 constructed in accordancewith a preferred embodiment of the invention. The sensor arrangement isintended to be used with an axle assembly in which the axle shafts 18,20 (shown in FIG. 1) are subject to lateral movement during operation.In particular, in axle assemblies used on heavy duty off-highway andon-highway vehicles, each axle shaft may extend between the differential12 (shown in FIG. 1) and an outboard planetary drive forming part of awheel hub (not shown). In at least some of these axle assemblies, thecenter line of the axle shaft changes when the direction of rotation isreversed. In other words, the center line of the axle shaft is in oneposition when the vehicle is moving in the forward direction and in adifferent position when the vehicle motion is reversed. Consequently,for these types of axle assemblies, a wheel speed sensor in which afixed gap between the fixed sensor and target or trigger cannot bemaintained.

As seen in FIG. 4, a portion 302 of the sensor 300 more commonlyreferred to as the pickup is secured to the axle housing 10a by a pairof bolts 310. The body of the pick-up extends through a bore in thehousing 10a and extends into abutting contact with the trigger or target312 formed on the axle. In the preferred embodiment, the trigger 312comprises a discontinuous, circular surface coupled to said wheel androtatable about the centerline of the axle. In the illustratedembodiment comprises a series of splines or teeth 312a formed ormachined on the periphery of the axle.

The pick-up 302 includes a fixed mounting portion 302a and areciprocally moveable portion 302b. The portion 302b is spring biasedand carries a sensing element 314 which, in the preferred embodiment,comprises a Hall effect cell located at the distal end of the moveableportion 302b. The cell 314 is conventional and is available from thirdparty suppliers such as Airpaxs instruments of Philips Technologieslocated in Cheshire, Conn. A coil spring 316 extending between the fixedportion 302a of the sensor body urges the Hall effect cell towardsabutting contact with the splines 312a. The movable portion 302b of thesensor includes a tubular section 318 which telescopes into the fixedportion 302b. A spring loaded seal 320 seals the interface between thetelescoping and fixed sections. A bushing 322, preferably made out of asteel/bronze alloy and which may be PTFE impregnated slidably supportsthe tubular support section.

A non-magnetic end cap 324 which serves as a follower is mounted on thedistal end of the tubular section 318. In the preferred embodiment, anend surface 324a of the follower is constructed of a wear-resistantmaterial such as bronze may be impregnated or coated with a low-frictionmaterial, such as delrin. Alternately, the entire end cap 324 may beformed of a non-metallic anti-friction material such as delrin. Theoutboard end of the pick-up preferably includes an electrical connector326 by which signal lines 328 extending between the sensor 30 and thecontroller 40 (see FIG. 1) are releasably connected.

Although the invention has been described in connection with left andright driven wheels of a vehicle, it should be understood that theprinciples of the invention can be applied to vehicles having front andrear driven wheels, i.e., four wheel drive vehicles having interaxledifferentials.

Although the invention has been described with a certain degree ofparticularity, it should be understood that those skilled in the art canmake various changes to it without departing from the spirit or scope ofthe invention as hereinafter claimed.

We claim:
 1. Apparatus for monitoring the rotational speed of a drivenwheel, comprising:a) a pick-up including a fixed body portion and amovable portion including a follower; b) a trigger including adiscontinuous, radially outer, circular trigger surface rotationallycoupled to said wheel such that rotation of said wheel produces rotationin said trigger surface about an axis; and, c) a member biasing saidfollower of said movable portion of said pick-up into abutting contactwith said radially outer trigger surface; d) said moveable portionmovable towards and away from said trigger surface and said triggersurface having a path of motion located intermediate said follower andsaid axis.
 2. The apparatus of claim 1 wherein said movable portion ofsaid pick-up includes a non-magnetic end cap defining said follower. 3.Apparatus for monitoring the rotational speed of a wheel driven by anaxle, comprising:a) a pick-up including a fixed body portion secured toan axle housing; b) a movable portion movable towards and away from saidaxle for driving said wheel, said axle having a centerline about whichsaid axle rotates; c) a radially outer trigger surface formed on saidaxle and moving in a circular path about said axle centerline; d) aresilient member biasing said movable portion of said pick-up intoabutting contact with said radially outer trigger surface, saidresilient element further operative to maintain contact of said movableportion with said trigger surface in the event said centerline of saidaxle changes position in relation to said axle housing.
 4. The apparatusof claim 3, wherein said radially outer trigger surface comprises aseries of splines formed on a periphery of a portion of said axle. 5.The apparatus of claim 4, wherein said movable portion carries a sensingelement which detects motion of said splines.
 6. The apparatus of claim4, wherein said movable portion includes a non-magnetic end cap whichserves as a follower and includes an end surface that is in abuttingcontact with said trigger surface.
 7. The apparatus of claim 6, whereinthe end surface is constructed of a wear resistant material.
 8. Theapparatus of claim 6, wherein said end cap is formed of a non-metallicanti-friction material.
 9. The apparatus of claim 8, wherein said endcap is formed of delrin.
 10. The apparatus of claim 3, wherein saidresilient member comprises a spring and said moveable portion includes afollower.
 11. A method for sensing the rotational speed of a drivenwheel, comprising the steps of:a) providing a pick-up including asensing element; b) providing a radially outer, trigger surface on adrive member coupled to said wheel; c) providing said pick-up with aportion that is movable towards and away from said radially outertrigger surface; d) urging said movable portion towards said triggersurface using a resilient element so that contact with said radiallyouter trigger surface is maintained in the event a centerline of saidwheel drive member changes position with respect to a fixed portion ofsaid pick-up.
 12. Apparatus for monitoring the rotational speed of adriven wheel, comprising:a) a pick-up including a fixed body portion anda movable portion mounting a sensing element; b) a trigger including acircular, radially outer trigger surface formed on a rotating drivemember coupled to said wheel such that rotation of said wheel producesrotation in said radially outer trigger surface about an axis of saiddrive member; and, c) a spring biasing an end surface of said movableportion of said pick-up into confronting engagement with said radiallyouter trigger surface such that said sensing element detects movement ofsaid trigger surface, said spring being operative to maintain saidconfronting engagement with said radially outer trigger surface.
 13. Awheel sensor for monitoring the rotational speed of a trigger having aradially outer surface that is rotatable about a centerline,comprising:a) a pick-up including a fixed body portion and a movableportion including a follower having an end surface extending obliquelywith respect to a line of action defined by said movable portion; b) amember biasing said follower of said movable portion of said pick-upinto abutting contact with said radially outer trigger surface; c) saidmovable portion movable towards and away from said radially outertrigger surface, such that said end surface maintains contact with saidradially outer trigger surface in the event said centerline of saidtrigger changes position with respect to the fixed body portion of saidpick-up.
 14. The wheel sensor of claim 13, wherein said end surface issubstantially orthogonal to a line of action defined by said movableportion.